1. Field of the Invention
The present invention relates to a driving method for optical modulation devices such as liquid crystal devices, and more particularly to a time-sharing driving method for liquid crystal device for use in optical modulation devices such as display elements or optical shutter arrays. Specifically, the present invention is concerned with a time-sharing driving method suitable for liquid crystal devices bistable liquid crystals.
2. Description of the Prior Art
Hitherto, liquid crystal display devices have been well known, wherein a group of scanning electrodes and a group of signal electrodes are arranged so as to intersect with each other and form a matrix, and liquid crystal compounds are filled into a space defined by these electrodes, thereby to form a large number of picture elements to display images or information. A time-sharing driving method is employed for driving these display devices. The time-sharing driving method is featured by sequentially and cyclically effecting selective application of address signals to a group of common electrodes serving as scanning electrodes, and effecting selective application of predetermined information signals in a parallel fashion in synchronism with address signals. However, the display devices and the driving method therefor have serious drawbacks referred to below.
Namely, it is difficult to obtain a high density of picture elements and a large image area. Most of liquid crystals which have been put into practice for display devices are TN (twisted nematic) type liquid crystals because they show a relatively high response speed and a small power dissipation among the conventional liquid crystals. These TN type liquid crystals are described for example by M. Schadt and W. Helfrich, "Voltage Dependent Optical Activity of a Twisted Nematic Liquid Crystal" Applied Physics Letters, Vol. 18, No. 4 (Feb. 15, 1971 pp. 127-128). In the liquid crystal of this type, molecules of nematic liquid crystal having a positive dielectric anisotropy, under application of no electric field form a twisted structure (helical structure) in the direction of the thickness of the liquid crystal layer and aligned or oriented parallel to each other at the both electrode surfaces. On the other hand, under application of the electric field, nematic liquid crystals having a positive dielectric anisotropy are aligned in the direction of the electric field, thereby making it possible to cause optical modulation. A display device having a matrix electrode structure may be constituted using a liquid crystal of this type. In this instance, a voltage higher than a threshold level required for orienting liquid crystal molecules in the direction perpendicular to the electrode surfaces is applied to a region (a selected point) where a scanning electrode and a signal electrode are concurrently selected, whereas no voltage is applied to a region (a non-selected point) where a scanning electrode and a signal electrode are not concurrently selected, and therefore, liquid crystal molecules maintain their stable alignment in parallel with the electrode surfaces. When linear polarizers arranged in the relationship of cross nicols, i.e., with their polarizing directions being substantially perpendicular to each other are provided on the upper and lower sides, respectively, of the liquid crystal cell, light does not transmit at the selected points but transmits at the non-selected points thereby making it possible to constitute an image device. However, when a matrix electrode structure is constituted, a certain electric field is applied to a region where a scanning electrode is not selected while a signal electrode is selected so-called "half-selected point"). If the difference between the voltage applied on the selected points and that applied on the half-selected points is sufficiently large, and the voltage threshold level required for orienting liquid crystal molecules in the direction perpendicular to the electrode surfaces is set to a voltage therebetween, the display devices operate normally. Actually, however, according as the number (N) of scanning lines increase, the time (duty ratio) during which effective electric field is applied on one selected point when the whole image area (one frame) is scanned, decreases in the proportion of 1/N. For this reason, the larger is the number of scanning lines, the smaller is the effective voltage which is a difference in voltage applied between selected points and non-selected points when repeatedly scanned. This leads to inherent drawbacks that image contrast is lowered and there occurs crosstalk. To overcome these drawbacks, the voltage averaging method, the two frequency driving method or the multiple matrix method have been previously proposed. However, none of these methods is sufficient to overcome the drawbacks stated above. Thus, it is the present state that the enlargement of image area or the densification of picture elements in respect to display devices is delayed because of the fact that it is difficult to sufficiently increase the number of scanning lines.
Meanwhile, a printer is proposed, wherein there is employed a system comprising means of changing electrical image signals into optical signals using a liquid crystal-shutter array, and irradiating optical signals to an electrophotographic photosensitive member thereby to form images. The liquid crystal-shutter array comprises microshutters of liquid crystal cells arranged in the form of an array and can selectively output transmitted lights by making use of electro-optical modulation of the liquid crystal as a microshutter.
The advantages given by the liquid crystal-optical shutter array are as follows:
1. When applied to an electrophotographic printer, the printer becomes small in size.
2. There is no mechanically movable part such as a polygon scanner as used in an LBP (Laser Beam Printer), resulting in no noise and small requirement in respect to the strict mechanical precision.
The fact that a liquid crystal-shutter array can offer these advantages leads to the possibility of affording improved reliability, light weight and reduced cost. However, there practically exist various difficulties, which will be explained by way of example.
Referring to FIG. 1, there is shown an example of a liquid crystal shutter array which would be most easily understood.
As shown in FIG. 1, there are provided openings 1 of the shutter, and the remaining parts except for the openings are usually masked so that light leakage does not occur. Liquid crystal is hermetically disposed between signal electrodes 3 (3a, 3b, 3c, 3d . . . ) provided at the inner wall surface of a base plate 2 and common electrodes 4 disposed opposite to signal electrodes 3. Each common electrode 4 is formed on a base plate (not shown) of a transparent plate of glass, plastics, etc. Likewise, the base plate 2 on which signal electrodes 3 are provided may be formed of a transparent plate of glass, plastics, etc. These base plates are spaced apart at a predetermined interval by a seal spacer (not shown) of a polyestel film, an eposy adhesive agent into which glass fiber is mixed or frit glass. Each of the signal electrodes 3 and the common electrode 4 may be formed of a transparent conductive film of indium oxide, tin oxide, ITO (indium oxide containing about 5% by volume of tin oxide), etc. Lead wires 5 and a lead wire 6 are drawn out from these electrodes 3 and 4, respectively, and are connected to a shutter array driving circuit (not shown).
FIG. 2 is a cross sectional view taken along the line II--II of the liquid crystal-optical shutter array illustrated as a plan view in FIG. 1. Referring to FIG. 2, an example of the operation of the shutter array is schematically shown.
The opening and closing operation of the shutter array is effected as follows. The orientation of liquid crystal 9 interposed between signal electrode 3 and common electrode 4 opposite thereto is controlled by selecting signal electrode 3 (3a, 3b, 3c, . . . ) to which voltage is applied. Thus, transmitted light T is determined in response to irradiated light I.
In FIG. 2, polarizing plates 7 and 8 are disposed in the arrangement of cross nicols, i.e. with their polarizing directions being substantially perpendicular to each other. The orientation treatment has been applied to the two base plates by a method such as rubbing in such a manner that the initial orientation direction forms an angle of 45.degree., respectively, with respect to the polarizing directions of polarizing plates 7 and 8. In this example, a liquid crystal having a positive dielectric anisotropy (P-type liquid crystal) is used as liquid crystal 9.
In this arrangement of the liquid crystal-shutter array, selected voltages are applied to signal electrodes 3a, 3b, 3c, . . . and common electrode 4 is earthed.
In the operation shown in FIG. 2, for example, a relatively large voltage is applied on the signal electrode 3b, and P-type liquid crystal molecules align substantially perpendicular to the surfaces of the cell . In this instance, the irradiated light I is not transmitted through the signal electrode 3b. On the contrary, as a relatively small voltage of zero or less than the threshold level is applied to signal electrodes 3a and 3c, the alignment or orientation of the P-type liquid crystal molecules corresponding to electrodes 3a and 3c is changed. Thus, the irradiated light I is transmitted as transmitted light T. In this operating method, by using a monochromatic light as the irradiated light I, a higher contrast is obtained. However, even if white light is used, a sufficient contrast can be obtained.
By repeatedly effecting the above-mentioned operation, the shutter array provides image signals to be photosensitive member, etc.
The above-mentioned liquid crystal driving method can be practised with the simplest structure, but has a drawback that the driving speed is low. This drawback is based on the fact that the change from the "off" state of the shutter, i.e. the state giving an orientation of the liquid crystal under high electric field to the "on" state is dependent solely upon natural reverting force to the initial orientation (obtained, e.g. by rubbing) of the liquid crystal under the zero electric field or weak electric field. Further, a serious drawback is pointed out in realizing low cost. Assume now that the openings are aligned in a manner shown in FIG. 1. If an attempt is made to design a shutter array with a density of picture elements of 10 dot/mm and a length corresponding to the lateral width of size A4, about 2,000 signal electrodes are required. Accordingly, about 2,000 drivers are required for driving respective signal electrodes.
This means that 40 integrated circuit (IC) chips for drivers are required when IC Chips having 50 pins are used, thus resulting in a limitation in reducing cost.
For this reason, another attempt is proposed to divide a common electrode into a plurality row electrodes. Thus, the plurality row electrodes are so arranged in combination with signal electrodes to form a matrix of electrodes, thereby reducing the nunber of signal electrodes. With respect to each row of the common electrodes, the opening and closing operation of the shutter is effected in a time-sharing manner. However, when such a configuration is applied to a liquid crystal shutter array, there is a possibility that light is transmitted not only from a row electrode at which the opening and closing operation of the shutter is effected, but also from another row electrode to be placed in shutter off condition, thus failing to give high performance.